This invention relates to an unlimited sliding ball spline assembly.
A machine basically consists of a linearly moving mechanism and a rotationally moving mechanism. Moving mechanisms for making these movements separately or together are employed in many machines and mechanical parts, for example, spindles for various types of machine tools, support posts or arms for industrial robots, precision index shafts, riveting machines, transfer machines, automatic carrying apparatuses, auto-loaders, and tire-molding machines. It has been increasingly demanded that ball splines to be used for the above purposes, which have highly-improved performance, become available.
Conventional unlimited sliding ball splines of this kind include, for example, an endless sliding ball spline shaft bearing (refer to U.S. Pat. No. 3,808,839) consisting of a spline bearing composed of an outer cylinder, a ball retainer provided on the inner side of the outer cylinder, and a plurality of balls held by the ball retainer in a plurality of endless passages, which are formed in an inner surface of the outer cylinder, in such a manner that the balls can be rolled in the endless passages; and a spline shaft supported on the plural balls in the spline bearing in such a manner that the spline shaft can be moved linearly and reciprocatingly in the axial direction thereof. Conventional ball splines of this kind also include a ball spline, which is provided with a mounting flange formed integrally with and at one end portion of an outer surface of an outer cylinder of a spline bearing similar to the above-mentioned spline bearing, to thereby furnish the ball spline with a capability of being mounted easily on a third member, such as a housing.
However, in order to form a compound moving mechanism for making linear, axial reciprocating movements and torque-transmitting rotational movements together, by using the former ball spline, it is necessary to prepare power-transmitting gears, a cylindrical member for mounting these gears thereon, and a bearing for mounting the moving mechanism on a housing. This causes an increase in the number of parts and manufacturing cost. Moreover, even when each part is processed at the highest possible accuracy, the processing errors in the parts are accumulated to make it difficult to increase the processing accuracy of the moving mechanism as a whole. Such a moving mechanism has many other problems; designing the moving mechanism is very troublesome, and forming it compactly is difficult.
Conventional ball splines including the latter ball splines, especially, an unlimited sliding ball spline provided in a place where it is used under severe conditions with a vibration impact load received thereon, and used in a place where it requires to be positioned at a high accuracy or have high-speed moving performance, require to have a high rigidity and a high processing accuracy. Accordingly, in order to manufacture an outer cylinder of a spline bearing in a conventional ball spline of this kind, very troublesome operations are required; such an outer cylinder is manufactured by subjecting a solid material to a turning process to form a thick-walled tubular body, broaching an inner surface of the tubular body to form ball guide grooves, ball-burnishing the loaded ball guide grooves, subjecting the resulting product to cementation hardening, subjecting the hardened product at its end portion to a turning process to form a flange thereon, ball-burnishing the resulting product again, grinding the guide grooves, and finally mounting the outer cylinder thus formed on a spline shaft to then grind an outer circumferential surface of the outer cylinder and the flange portion thereof. Especially, if an outer cylinder is subjected to a turning process before it has been cementation-hardened, to form a mounting flange thereon during the production of an outer cylinder having a mounting flange at one end portion of the outer surface thereof, strain occurs in the outer cylinder due to the differences between the thicknesses of various parts thereof when it is subjected to a cementation hardening step. Accordingly, it is necessary that an outer cylinder be subjected to a turning process for the formation of a mounting flange after it has been cementation-hardened. In fact, a turning process for forming a mounting flange on an outer cylinder takes very much time and costs a great deal.
Let the spotlight of attention be focused upon the movements of balls in an unlimited sliding ball spline. The balls are circularly moved as they are rolled in an endless, unlimited passage defined by guide grooves in an outer cylinder and guide grooves and elongated grooves in a ball retainer fitted in the outer cylinder. The outer cylinder is ideally expanded outward at both end portions thereof to allow the balls in the endless passage to be rolled smoothly from a no-load region to a load region, or from a load region to a no-load region. The effect thus obtained is called "crowning effect".
The guide grooves provided in such a conventional ball spline are polished with a rod type oil-stone, i.e. subjected to a so-called crowning process, after they have been ground, so as to allow the ball spline to display excellent crowning effect. However, the crowning process cannot help being carried out manually. In addition, it is necessary that the six or more guide grooves formed in the inner surface of the outer cylinder be polished uniformly with attention paid to the size of a hollow in the outer cylinder so as to fit a spline shaft therein properly. This causes the efficiency of the crowning process to decrease to an extremely low level. Eliminating the above-mentioned inconveniences has been a serious problem.
Known ball splines of this kind further include a ball spline provided with a bearing, which is used to roll-guide linear, axial reciprocating movements, rotational movements, and a combination of these movements, of moving mechanisms, and which consists of a tubular outer cylinder, a ball retainer fitted in the outer cylinder and having a length smaller than that thereof, and a plurality of balls held in the ball retainer in such a manner that a load of a shaft is applied equally to the balls.
However, in such a conventional bearing, a stroke of a linear, axial reciprocating movement of a spline shaft is limited to a distance, which is two times as long as a distance, for which the ball retainer can be moved in the outer cylinder. In the case where a large stroke of a spline shaft is required, it is necessary that the length of the outer cylinder be increased in accordance with the same stroke. Accordingly, in a machine, in which it is difficult to mount thereon a ball spline having a bearing with a long outer cylinder, a stroke of a spline shaft cannot be increased. Even when a machine permits a ball spline having a bearing with a long outer cylinder to be mounted thereon, the dimensions of the machine are unnecessarily increased.